NEWPORT, Ore. - A team of Oregon State University scientists has solved the mystery behind an unusual swarm of earthquakes that occurred off the Oregon coast in the spring of 2008 - a series of faults in the Juan de Fuca plate that they didn't know existed.
The discovery of these faults about 140 miles off the central Oregon coast, in association with the earthquake activity, suggests that the tectonic plate off the Oregon shore is still actively deforming, said Robert Dziak, an OSU marine geologist who works at the university's Hatfield Marine Science Center in Newport.
"This pattern of earthquakes demonstrates that the Juan de Fuca plate is continually moving and converging with North America at the Cascadia Subduction Zone," Dziak said. "It isn't clear if the swarms that occurred in 2008 represent normal stress release within the plate, or if they are from deformation related to the Cascadia Subduction Zone. We simply don't yet know."
Most of the earthquakes were of magnitude 3.0 to 4.0, the scientists said, but there were a handful that exceeded magnitude 5.0. Few, if any, of these earthquakes would be felt on shore, Dziak said, because they originate offshore within the deep ocean floor. The Cascadia Subduction Zone is of particular interest because the region has experienced several enormous earthquakes over the past 10,000 years - the last of which occurred about 300 years ago.
The intense earthquake swarm last year began on March 30 about 140 miles southwest of Newport and was one of the more unusual events detected by Dziak and his colleagues in 17 years of monitoring using sensitive undersea hydrophones. The swarm was considered unusual because it began inside the Juan de Fuca plate and not along the boundary between the Juan de Fuca and Pacific plates, where most earthquake activity takes place.
Then after 10 days, the swarm stopped - but not for long. Three distinct clusters of quakes soon followed, beginning with a series of small tremors along the Blanco Transform Fault - the boundary between the two plates - and concluding with a frenzy of seismic activity along the Gorda Ridge, which produced more than 1,000 earthquakes in just five days. This swarm was of special interest to scientists, not just because of the sheer volume of quakes, but because of its proximity to an eruption on the seafloor discovered in 1996.
Dziak said the two-month swarm represented a plate motion event, beginning within the Juan de Fuca plate, then moving east and south, and finally culminating in seafloor spreading activity that likely produced magma intruding beneath the seafloor.
"We were able to monitor the spatial progression of the swarm within the plate and along its boundaries," Dziak said, "but we don't yet completely understand how they are related and what triggers the sequence. But it is interesting that the stress release within the plate could trigger swarms of earthquakes on the plate boundaries."
During the two-month spree last spring, the OSU scientists recorded more than 1,600 earthquakes using an array of hydrophones called the Sound Surveillance System (SOSUS), made available by the U.S. Navy. This SOSUS network originally was used during the decades of the Cold War to monitor submarine activity in the northern Pacific Ocean. As the Cold War ebbed, these and other unique military assets were offered to civilian researchers performing environmental studies, Dziak said.
When the researchers first detected the swarm, they mobilized the OSU research vessel Wecoma on a trip led by Ron Greene to take water samples in the earthquake zone and look at the chemical signature of the water for signs of volcanic activity. In September, Hatfield researcher Susan Merle returned aboard the R/V Melville and performed a multi-beam sonar survey to produce new maps of the seafloor and it was during this cruise that the new fault system was discovered.
"From aboard the ship, we discovered one area where there was a 20-meter displacement of the seafloor and deformed sediments, which is a direct indication of faulting," said Merle, a senior faculty research assistant at OSU. Merle, Greene and Dziak are all affiliated with the Cooperative Institute for Marine Resources Studies (CIMRS), a joint OSU/NOAA venture.
An additional high-resolution seafloor survey of the eastern Blanco Transform Fault was performed last summer by the NOAA ship Okeanos Explorer.
This isn't the first time the researchers have recorded earthquake swarms off the Oregon coast. There have been a total of eight swarms over the past dozen years, Dziak said, the first seven of which likely were the result of volcanic activity on the Juan de Fuca and Gorda ridges. The 2008 swarm originated within the plate, where the newly discovered faults lie and affected a large area of the plate and its boundaries.
"The discovery sheds some new light on the structure and seismic processes of the region," Dziak said, "and suggests that deformation within the plate and earthquakes along its boundaries may be more interrelated than we though. It also underscores the importance of having ships available to go to normally inaccessible areas of the deep-ocean for research that addresses societal concerns."